WIP - fastly support

[chrisradek]

Current state of PR

This PR adds support for running the edge sdk on Fastly Compute@Edge. Previously this was only tested and ran on Cloudflare Workers.

Cloudflare Workers versus Fastly Compute@Edge

Feature	Cloudflare Workers	Fastly Compute@Edge
Language support	JavaScript	JavaScript compiled to WASM
Streaming HTML modification	HTMLRewriter	Hand-crafted solution with transform streams or buffering
Geo data	request.cf	event.client.geo
Request construction	Supports passing in full Request as 2nd param	Will not use request.body if passed as 2nd param
fetch	works similar to node.js	requires passing in backend field unless dynamic backends are enabled
automatic decompression	gzip/deflate, maybe brotli?	gzip if passed as request option

Currently working in PR

• Proxy origin
• AJS injection
  • Currently buffers whole HTML file, need to take advantage of TransformStreams
• AJS assets proxy
• TAPI proxy
• reset endpoint
• server-side cookies
• writeKey redaction

Features still in progress

• Edge context
  • geo data exists in fastly, but not on the request object
• profiles API
• Engage incoming webhook
• client side traits
• edge variations

Testing progress

Created a new edge-sdk-integration-tests package to run integration tests in as close to the real environments as possible (without actually deploying services).

This uses wrangler v3, which utilizes workerd, for cloudflare workers, and the fastly cli's fastly compute serve command for building/running fastly services locally. Tests currently all pass locally, but need to update CI so that the fastly CLI is available to use.

Notes

Will probably break this up into smaller PRs once it's ready for review to keep them somewhat sane in size.

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